Cold-working magnesium alloy tubes



June 19, 1945. H. w. SCHMIDT 7 COLD WORKING MAGNESIUM ALLOY TUBES Filed Dec. 17, 1941 I Die Ma mes/um bare .Oranah? 6/70 I in d/ame/er' INVENTOR.

l/erb r/ n4 xchmw BY v JZTOLUE VS Patented June 19, 1945 I 2,378,729 COLD-WORKING MAGNESIUM ALLOY TUBES Herbert w. Schmidt, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Application December 17, 1941, Serial No. 423,374

Claims.

- This invention relates to the cold working of tubes formed of magnesium and magnesium-base alloys. Y

In certain of the industrial applications of tubing or pipe formed of magnesium or magnesiumbase alloys, especially in the use of the larger sizes for temporary oil well casings, the tubing is subjected externally to comparatively great hydraulic pressures, which, of course, tend to collapse the tubing inwardly. For such purposes, it is desirable that the tubes possess an extremely high compressive yield strength, so as to resist external pressures without appreciable distortion. In general, strength values considerably in excess of those exhibited by magnesium-alloy tubing as conventionally fabricated are required. It will be evident, however, that any process for producing these unusual compressive strengths in magnesium-base alloy tubing should, ii possible, not affect adversely other important physical properties of the metal, particularly, its ductility, as measured by elongation under tension. Unfortunately, no process of this character has heretofore been known.

It is therefore an object of the present invention to provide a process of working tubes formed of magnesium or magnesium-base alloys to increase the compressive strength thereof which does not appreciably decrease the ductility of the metal and which in many instances actually increases this and other physical properties of the metal.

According to the invention, the compressive strength of tubes formed of magnesium and plastically deformable magnesium-base alloys may be markedly increased by subjecting the tubes to cold working by applying substantially radial compressive forces to the outer surface thereof to an extent suflicient to effect a small reduction in the tube diameter. The resulting inward deflection of the tube wall changes the internal structure of the metal in such manner as to raise its compressive strength without appreciably decreasing its ductility, as measured by the elongation under tension. In most instances, the process of the invention actually raises the elongation of the metal, and also improves the tensile properties of the tubing.

In general, the new method is carried out without the use 'of a mandrel inside the tube, 1. e., without internal support for the tube, working being accomplished by applying the substantially radial compressive forces to the outer surface of the tubing progressively along the len th thereof.

4 In this way, the diameter of the tubing is reduced without decreasing the wall thickness of the tubing, the working usually being conducted in such manner as actually to increase somewhat the wall thickness. The tubing may lengthen very slightly during working but the change in length rarely exceeds a few per cent.

The process, which is ordinarily applied to tubing in the as-extruded state, is conducted at a cold-working temperature, i. e., below the recrystallization temperature of themetal forming the tube. For most magnesium-base alloys, temperatures below 400 or 500 F. are required, room temperature being customary. The compressive forces are preferably applied in suflicient magnitude to effect a reduction in diameter of the tube between about 2 and about 7 per cent. Lesser reductions do not produce the desired properties to a satisfactory degree, and reductions much over 7 per cent tend to reduce the ductility of the metal or even to cause cracking.

Application of the substantially radial compressive forces to the tube may be efiected by any of a variety of mechanical operations. A particularly satisfactory procedure is to force the tube through a, circular die having a working diameter from 2 to 7 per cent less than that of the unworked tube. Conveniently, the tube is pointed and then drawn through the die without a mandrel, using a draw-bench, according to the operation commonly known as die-sinking. Alternatively, the tube may be pushed through the die by applying longitudinal pressure toward the die to that portion of the tube which has not yet entered the die. The reduction in diameter is best effected in a single pass through the die, although reductions in two Or more passes are possible. The use of lubricants, particularly sulfurized tallow, reduces the power required in die-reducing, although the tube may be worked dry.

The accompanying self-explanatory drawing illustrates diagrammatically in cross-section the die-sinking of a magnesium-base alloy tube according to the invention.

While die-reducing is perhaps the most advantageous method of applying the substantially radial compressive forces required to produce the reduction in diameter according to the invention, other methods are operable. For instance, the tube may be subjected to repeated rapid hammer blows in a transverse direction around its entire periphery and throughout its length, as by feeding the tube slowly through a conventional swaging machine or a l lo-forming machine. Alternatively, the tube may be rolled on forming rolls adapted to effect a 2 to per cent reduction in diameter of the tube.

The following examples will serve to illustrate the invention, but are not to be construed as limiting its scope:

Example 1 billet of the alloy through a mandrel die,.in accordance with conventional practice. Certain of the tubes were then slotted at one end to facilitate drawing and were cold-drawn without a mandrel through a circular die having a working diameter of 1.920 inches, using a drawing pressure of 6500 pounds. The reduction in diameter so effected was 3.76 per cent; the wall thickness increased 3.28 per cent. Both the asextruded and the cold-drawn tubing were then tested by standard methods to determine the physical properties thereof. In addition, the collapse pressures of the tubes were measured by capping the tubes, inserting them in a closed vessel provided with means for venting the interior of each tube, and applying increasi hydraulic pressure to the outside of the tubes until they failed by collapsing inwardly. The properties so measured were:

Extruded tubes of Dowmetal F, prepared as in Example 1, and having an outside diameter of 1.993 inches and a wall thickness of 0.183 inch were pushed at room temperature through a die having a working diameter of 1.881 inches by applying force longitudinally to the tube to cause it to pass through the die, no mandrel being used. The reduction in diameter obtained was 5.90 per cent; the tube wall thickness increased 6.0 per cent. The properties of the tubes, before and after forcing through the die, were then measured as in Example 1, being:

As-extruded Cold-worked Compressive yield lbs. per sq. in.. 10, too 25, 600 stre ng.

Tensile yield do.-.- 20,700 28,460

strength. Elongation in 2 in --per cent.- 11. 7 12. Collapse pressure -.lbs. per sq. in-. 2, 798 5, 000

Example 3 Extruded tubes of Dowmetal F, prepared as in Example 1, and having an outside diameter of 5.61 inches and a wall thickness of 0.345 inch were pushed at room temperature through a die having a working diameter of 5.35 inches. The reduction in diameter so accomplished was asveyas and after forcing through the die, were then measured as in Example 1. The results were:

Extruded tubes of Dowmetal F, prepared as in Example 1, and having an outside diameter of 1.752 inches and a wall thickness of 0.12 inch, were passed at room temperature through a ,swaging machine to effect a reduction in diameter of 6.8 per cent. The properties of the tubes,

both before and after swaging, were measured as in Example 1. The values observed were:

As-extruded Swaged Compressive yield strength, long lbs. per sq. in-- 14, 550 27, 100 Tensile field strength do... 36, 700 45, 300 Elongat n in 2 in per cent- 14. 0 l3. 0 Collapse pressure lbs. per sq. in 1, 780 4, 000

From the foregoing examples, it will be apparent that the process of the invention effects a marked increase in the collapse pressure of magnesium-base alloy tubing without appreciably decreasing the ductility thereof, in some instances even increasing this latter. These results are seen to be distinctly unusual when it is considered that tubes formed of metals other than magnesium and magnesium-base alloys invariably suffer great loss of ductility in die-sinking operations and that heretofore magnesiumbase alloys have been thought always to undergo a considerable decrease in ductility during coldworking.

Other modes of applying the principle of the invention may be employed instead of those explained, change being made as regards the details set forth, provided the step recited in any of the following claims or the equivalent thereof be employed.

1 claim: 1

1. In a method of working tubes formed of magnesium and plastically deformable magnesium-base alloys to increase the compressive strength thereof without materially decreasing the ductility of the metal, the step which comprises subjecting the tube to cold working by applying substantially radial compressive forces to the outer surface thereof to an extent sufflcient to effect a small reduction in the tube diameter.

2. In a method of working tubes formed of magnesium and plastically deformable magnesium-base alloys to increase the compressive strength thereof without materially decreasing the ductility of the metal, the step which comprises progressively applying substantially radial compressive forces to the outer surface of the tube throughout the length thereof to effect a reduction in tube diameter of from about 2 to about 7 per cent, while maintaining the tube at a cold-working temperature, the said operation being carried out without the use of a mandrel.

,3. In a method of working tubes formed of magnesium and plastically deformable magnesium-base alloys to increase the compressive strength thereof without materially decreasing 4.6 per cent. The properties of the tubes, before the ductility of the metal, the step which comprises die-sinking the tube at a cold working temperature to an extent sufficient to eflect a small reduction in the tube diameter.

4. In a method of working tubes formed of magnesium and plastically deformable magnesium-base alloys to increase the compressive strength thereof without materially decreasing the ductility of the metal, the step which comprises forcing the tube through a circular die having a working diameter of from about 2 to about 7 per cent less than the diameter of the unworked tube, the said operation being car- 10 diameter.

ried out without a mandrel while the tube is at a cold working temperature.

5. In a method of working tubes formed of magnesium and plastically deformable magnesium-base alloys to increase the compressive strength thereof without materially decreasing the ductility of the metal, the step which comprises swaging the tube at a cold working temperature to effect a small reduction in the tube HERBERT W. SCHMIDT. 

